Mirror based microelectromechanical systems and methods

ABSTRACT

Unlike most MEMS device configurations which simply switch between two positions in many optical devices the state of a MEMS mirror is important in all transition positions. It may determine the characteristics of an optical delay line system and by that an optical coherence tomography system in one application and in another the number of wavelength channels and the dynamic wavelength switching capabilities in the other. The role of the MEMS is essential and it is responsible for altering the paths of the different wavelengths in either device. It would be beneficial to improve the performance of such MEMS and thereby the performance of the optical components and optical systems they form part of. The inventors have established improvements to the design and implementation of such MEMS mirrors as well as optical waveguide technologies to in-plane optical processing as well as the mid infrared for optical spectroscopy.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit as a continuation of U.S. patentapplication Ser. No. 15/124,259 filed on Sep. 7, 2016, which itselfclaims the benefit of priority as a 371 National Phase Application ofPCT/CA2015/000136 filed Mar. 9, 2015, which itself claims the benefit ofU.S. Provisional Patent Application 61/949,474 filed Mar. 7, 2014, theentire contents of all being incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to microelectromechanical systems and moreparticular to designs and enhancements for MEMS mirrors and opticalcomponents exploiting such MEMS mirror elements.

BACKGROUND OF THE INVENTION

Wavelength division multiplexing (WDM) has enabled telecommunicationservice providers to fully exploit the transmission capacity of opticalfibers in their core network. State of the art systems in long-haulnetworks now have aggregated capacities of terabits per second.Moreover, by providing multiple independent multi-gigabit channels, WDMtechnologies offer service providers with a straight forward way tobuild networks and expand networks to support multiple clients withdifferent requirements. At the same time these technologies have evolveddown through the local area networks to the subscriber access networksand into data centers to support the continuing inexorable demand fordata. In order to reduce costs, enhance network flexibility, reducespares, and provide reconfigurability many service providers havemigrated away from fixed wavelength transmitters, receivers, andtransceivers, to wavelength tunable transmitters, receivers, andtransceivers as well as wavelength dependent add-drop multiplexer, spaceswitches etc.

At the same time, improvements in imaging technology have had a greatimpact on modern medicine. Imaging is a powerful tool that allowsnon-invasive diagnostics, helps to plan and direct surgicalinterventions, and facilitates treatment monitoring. One emergingimaging techniques is Optical Coherence Tomography (OCT), which canprovide high-resolution 3D images. This technique is a non-invasive andnon-contact technology. In the last decade, optical coherence tomographyhas found applications in several medical fields, includingophthalmology, dermatology, cardiology, dentistry, neurology, andgastroenterology.

At first sight, the provisioning of wavelength tunable transmitters,receivers, and transceivers for optical telecommunications may seem tohave little in common with medical imaging systems operating at videoframe rates with cycling speed rates of over 1 kHz and delay ranges ofmore than 3.33 ps to support millimeter depth penetration using OCT.However, in both applications the requirements for smaller footprint,improved performance, and reduced cost have led to the adoption ofmonolithic optical circuit technologies, hybrid optoelectronicintegration, and exploitation of technologies such asmicroelectromechanical systems (MEMS).

A common MEMS element to both is a MEMS mirror capable of deflectionunder electronic control. However, unlike most MEMS deviceconfigurations where the MEMS is used to simply switch between twopositions in these devices the state of MEMS is important in alltransition positions. Additionally, in the optical system designsdescribed according to embodiments of the invention the MEMS mirrorrotates in-plane. The characteristics of the MEMS determines thecharacteristics of the whole optical delay line system and by that theOCT system in one and in the other the number of wavelength channels andthe dynamic wavelength switching capabilities in the other. The role ofthe MEMS is essential and it is responsible for altering the paths ofthe different wavelengths in either device.

Accordingly, it would be beneficial to improve the performance of suchMEMS and thereby the performance of the optical components and opticalsystems they form part of. Beneficially, the inventors have establisheda range of improvements to the design and implementation of such MEMSmirrors as well as optical waveguide technologies supporting theextension of these device concepts in the mid-infrared for opticalspectroscopy for example.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

SUMMARY OF THE INVENTION

It is an object of the present invention to mitigate limitations in theprior art relating to microelectromechanical systems and more particularto designs and enhancements for MEMS mirrors and optical componentsexploiting such MEMS mirror elements.

In accordance with an embodiment of the invention there is provided adevice comprising a microelectromechanical element, themicroelectromechanical element having at least a front surface and aback surface, and an optical circuit disposed adjacent to themicroelectromechanical element having a coupling surface having aprofile matching the front surface.

In accordance with an embodiment of the invention there is provided adevice comprising:

-   a microelectromechanical element, the microelectromechanical element    having at least a front surface and a back surface;-   an optical circuit disposed adjacent to the microelectromechanical    element having a coupling surface having a profile matching the    front surface; and-   a linear microelectromechanical actuator coupled to the    microelectromechanical element.

In accordance with an embodiment of the invention there is provided adevice comprising:

-   a microelectromechanical element, the microelectromechanical element    having at least a front surface and a back surface and comprising a    predetermined portion formed from a predetermined ceramic material;    and-   an optical circuit disposed adjacent to the microelectromechanical    element having a coupling surface having a profile matching the    front surface, the optical circuit comprising a waveguide having a    core formed from the predetermined ceramic material.

In accordance with an embodiment of the invention there is provided adevice comprising:

-   a microelectromechanical element comprising a predetermined portion    of a microelectromechanical device having a pivot point and    performing rotation around the pivot point under actuation; and-   an anchor spring attached to a predetermined portion of the    microelectromechanical element, wherein the anchor spring engages    against a predetermined portion of the microelectromechanical device    once the-   microelectromechanical element has rotated a predetermined angle.

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the attached Figures, wherein:

FIG. 1 depicts fixed and tunable semiconductor optical sources accordingto the prior art together with a tunable semiconductor optical sourceaccording to an embodiment of the invention;

FIG. 2 depicts a tunable semiconductor optical source according to anembodiment of the invention;

FIG. 3A depicts an optical coherence tomography (OCT) system togetherwith the design and performance of a polarization independent reflectingcoating for a silicon MEMS according to an embodiment of the invention;

FIG. 3B depicts a tunable optical delay line for an OCT systemexploiting a MEMS mirror according to an embodiment of the invention andits performance;

FIG. 4A depicts a MEMS mirror and tunable optical delay line componentsaccording to embodiments of the invention;

FIG. 4B depicts the tunable delay line performance for the tunableoptical delay line components according to the embodiments of theinvention depicted in FIG. 4A;

FIG. 5 depicts the tunable delay line performance for the tunableoptical delay line components and their designs according to theembodiments of the invention;

FIG. 6 depicts optical spectrometer designs exploiting MEMS mirrorsaccording to embodiments of the invention;

FIG. 7 depicts optical waveguide and MEMS design variants according toembodiments of the invention;

FIG. 8 depicts schematics for silicon nitride core waveguide basedcircuits according to embodiments of the invention and their resultingoptical channel counts with varying maximum rotation angle of the MEMSmirror;

FIG. 9A depicts inter-waveguide coupling strength for silicon nitridecore waveguides as a function of inter-waveguide gap for two differentcore waveguide thicknesses;

FIG. 9B depicts Bragg grating length versus grating bandwidth forsilicon nitride core waveguides;

FIGS. 10 to 13 depict an exemplary process flow for manufacturing aBragg grating waveguide array and a tunable MEMS mirror according to anembodiment of the invention;

FIG. 14 depicts silicon rib waveguide design and simulations for anisolated waveguide and waveguides within an array;

FIG. 15 depicts schematics for silicon rib waveguide based circuitsaccording to embodiments of the invention and their resulting opticalchannel counts with varying maximum rotation angle of the MEMS mirror;

FIG. 16 depicts Bragg grating length versus grating bandwidth forsilicon rib waveguides with varying rib depth;

FIGS. 17 to 20 depict an exemplary process flow for manufacturing aBragg grating waveguide array and a tunable MEMS mirror according to anembodiment of the invention;

FIG. 21 depicts a semi-circular MEMS mirror (SC-MEMSM) and actuatoraccording to an embodiment of the invention;

FIG. 22A depicts a SC-MEMSM and actuator according to an embodiment ofthe invention;

FIG. 22B depicts a detail of the actuator for the SC-MEMSM depicted inFIG. 21A;

FIG. 23 depicts SC-MEMSMs and actuators according to embodiments of theinvention;

FIG. 24 depicts simulated and measured rotation angle versuselectrostatic actuator voltage for the SC-MEMSM and actuator designsaccording to embodiments of the invention as depicted in FIGS. 21, 22A,and 23;

FIG. 25 depicts an optical micrograph of a SC-MEMSM and actuatoraccording to an embodiment of the invention as depicted in FIG. 21 underbias at 140V and the experimental rotation angle versus electrostaticactuator voltage for the design;

FIG. 26 depicts a SC-MEMSM and actuator with an additional linearactuator to adjust the separation of the SC-MEMSM from the adjacentstructure according to an embodiment of the invention; and

FIG. 27 depicts an alternate SC-MEMSM design according to an embodimentof the invention with rotational actuator in combination with gapactuator to adjust the SC-MEMSM gap and latching/locking actuators tomaintain SC-MEMSM position once set;

FIG. 28 depicts an example of a gap actuator for SC-MEMSM devicesaccording to an embodiment of the invention;

FIG. 29 depicts an example of a latching actuator for SC-MEMSM devicesaccording to an embodiment of the invention;

FIG. 30 depicts an example of a latching lock for a latching actuatorfor SC-MEMSM devices according to an embodiment of the invention;

FIG. 31 depicts the simulated rotation angle versus electrostaticvoltage for a SC-MEMSM design such as depicted in FIG. 27;

FIG. 32 depicts the displacement versus electrostatic voltage for aSC-MEMSM gap actuator such as depicted in FIG. 28;

FIG. 33 depicts the displacement versus electrostatic voltage for aSC-MEMSM latching structure such as depicted in respect of FIG. 29 forlatching the rotation angle of MEMS mirror; and

FIG. 34 depicts the displacement versus electrostatic voltage for aSC-MEMSM latching lock such as depicted in respect of FIG. 30 forlocking the latch mechanism allowing it to maintain position withoutapplied electrostatic control;

FIG. 35 depicts design variants of an anchor spring according toembodiments of the invention to reduce MEMS pull-in; and

FIG. 36 depicts an optical micrograph of a fabricated SC-MEMSM asdepicted schematically in FIG. 27 employing rotator MEMSM actuator, gapcloser actuator, latching actuator and latching locks.

DETAILED DESCRIPTION

The present invention is directed to microelectromechanical systems andmore particular to designs and enhancements for MEMS mirrors and opticalcomponents exploiting such MEMS mirror elements.

The ensuing description provides exemplary embodiment(s) only, and isnot intended to limit the scope, applicability or configuration of thedisclosure. Rather, the ensuing description of the exemplaryembodiment(s) will provide those skilled in the art with an enablingdescription for implementing an exemplary embodiment. It beingunderstood that various changes may be made in the function andarrangement of elements without departing from the spirit and scope asset forth in the appended claims.

1. APPLICATIONS

1A: Wavelength Tunable Optical Source

As noted supra wavelength tunable optical sources and/or receivers havesignificant benefit in the provisioning of transmitters, receivers, andtransceivers within todays optical communication networks and evolvingrequirements for optical networks with dynamic wavelength allocation,reduced installation complexity, single line card designs, andreconfigurability. Within the prior art several approaches have beenemployed to date and whilst these have demonstrated high performancetransmitters they suffer limitations such as assembly complexity,achievable performance, and cost. Two such prior art approaches aredepicted in second and third images 100B and 100C respectively incomparison to a standard fixed wavelength laser source in first image100A.

In first image 100A a fixed wavelength laser source is depicted in adual-in line (DIL) package configuration 115 and comprises monitorphotodiode (not identified for clarity) and laser diode die 111 mountedupon a chip carrier 112 which comprises a thermistor (not identified forclarity) for monitoring the temperature as the laser diode die 111 has afast wavelength versus temperature profile. The output of the laserdiode die 111 is coupled via an optical lens—optical isolator assembly113 such that is focused at a location 113 wherein the optical fiberwithin a ferrule assembly 114, for example, is positioned and assembledto couple the optical signal to the network via optical fiber pigtail116. The laser diode die 111 may, for example, be a distributed feedback(DFB) laser, a distributed Bragg reflector (DBR) laser or a monolithicexternally modulated DFB laser.

Accordingly, in second image 100B a wavelength settable transmitterassembly prior to optical fiber pigtailing and sealing is depicted. Asshown the assembly comprises a laser array 121, MEMS switch array 122,monitor photodiode 123 and wavelength locker 124. The wavelength locker124 provides a means of locking the laser array 121 to a predeterminedgrid, such as 100 GHz C-band grid of long-haul telecommunications around1550 nm. Accordingly, the laser array 121 comprises an array of opticalsources monolithically integrated into the same semiconductor die, e.g.40 DFB lasers. The provisioning of the selected wavelength for thetransmitter is determined by the provisioning of electrical drivecurrent to the appropriate DFB laser within the laser array 121 and theswitching of the appropriate MEMS switch element within the MEMS switcharray 122. As such the approach is costly in that not only must amonolithic indium phosphide (InP) M-channel DFB laser array beimplemented but also an array of M MEMS switches. Accordingly, in someinstances the free-space optical interconnect from the laser array 121to optical fiber (not depicted for clarity) is replaced by a wavelengthdivision multiplexer, such as an array waveguide grating (AWG) on thesame die as the laser array 121.

Third image 100C depicts an alternate wavelength tunable transmitterexploiting a an external cavity laser (ECL) configuration wherein ratherthan the laser diode die having two high reflectivity facets to supportthe required cavity oscillation to provide gain within the semiconductordevice the laser diode die has one or no high reflectivity facets andforms a resonant optical cavity with one or two external mirrors. Inthis instance a single external mirror 131 is employed in conjunctionwith a semiconductor optical amplifier (SOA) die 132 that has a highreflectivity facet towards the optical fiber pigtail 135 and a lowreflectivity facet towards the external mirror 131. The resultant laseroutput is coupled from the SOA die 132 to the optical fiber pigtail 135via isolator 133 and lens 134. In this instance the external mirror 132is a tunable Fabry-Perot cavity filter 131 that provides for wavelengthdependent reflectivity such that the output of the assembly iswavelength specific according to the settings of the tunable Fabry-Perotcavity filter 131 allowing the emission wavelength to be adjusted.However, the characteristics of the source are now defined by thequality of the Fabry-Perot cavity filter, which even when implementedusing a MEMS construction does not achieve the sidelobe rejection of theDFB approaches.

Accordingly, it would beneficial to provide a tunable wavelengthtransmitter which can be fabricated at reduced cost commensurate withthe pricing expectations of telecom system providers and telecomoriginal equipment manufacturers (OEMs) for high volume generalizeddeployment within optical access networks, local area network, and datacenters for example. Accordingly, the inventors have established ahybrid circuit implementation exploiting an ECL configuration utilizingan InGaAsP SOA, for 1310 nm or 1550 nm wavelength ranges, in conjunctionwith a silicon MEMS wavelength selective reflector (MEMS-WSR). Asdepicted in fourth image 100D the approach exploits a silicon basedMEMS-WSR which comprises a coupling region 144 for coupling between theSOA 145, a tunable MEMS mirror 141, and an array of Bragg reflectors143. The optical signals are coupled between the coupling region 144 andthe array of Bragg reflectors 143 by a planar waveguide region 142wherein the diverging optical signals from the Bragg reflector 143 arere-focussed by the tunable MEMS mirror 141. Accordingly, as depicted infirst and second schematics 150A and 150B the wavelength operation ofthe ECL is therefore controlled by the routing selection of the mirror141 to a selected Bragg grating within the array of Bragg reflectors143.

Referring to FIG. 2 an alternate configuration 200 for an ECL 260exploiting silicon photonics and MEMS is depicted in first and secondschematics 200A and 200B respectively. Accordingly, an optical gainelement 210 is coupled via a coupling region 220 to planar waveguideregion 240. The diverging optical signal from the optical gain element210 is coupled to a selected Bragg grating within an array of Braggreflectors 250 via the planar waveguide region 240 and MEMS mirror 230wherein the design of the MEMS mirror 230 is such that the opticalsignal is coupled and re-focussed to the plane of the waveguides formingpart of the Bragg gratings within the array of Bragg reflectors 250.

It would be evident that in addition to wavelength tunable transmittersthe approach of a MEMS mirror in conjunction with an array of Braggreflectors may also form part of wavelength tunable receivers,reconfigurable optical add—drop multiplexers (ROADMs), wavelengthselective optical switches, and other wavelength selective structures,for example.

1B: Integrated Continually Tunable Optical Delay Line

As outlined supra one of the Optical Coherence Tomography approaches isTD-OCT where a reference light signal is scanned with a variable delayand then compared with the light reflected back from the sample tomeasure the time of flight. A schematic representation of a basic TD-OCTsystem is presented in FIG. 3 in schematic 300A. As depicted the opticaloutput of a broadband source 305 is coupled via circulator 310 to apassive splitter 315 wherein it is split in two: the first part is usedas a reference and propagates to a variable delay line 325 which iscontrolled via a driver 330; whilst the other part is used to scan thesample 320. The light reflected from the sample is combined with thereference and the resulting interference is captured with aphotodetector 335. The resulting photodetector 335 output is coupled viaa de-modulator 340 to a controller 345. The interference patterngenerated at the photodetector 335 is used to generate an image alongthe depth of the sample 320. The variable delay line 325 is an essentialcomponent in TD-OCT systems as it defines the maximum speed and depth ofthe scans.

The new optical delay line system is a miniature and new designedversion of the bulk Fourier domain optical delay line system as knownwithin the prior art, see for example Rollins et al. in “In Vivo VideoRate Optical Coherence Tomography” (Optics Express, Vol. 3, No. 6,21914). Within an embodiment of the invention an optical signal fromintegrated optical waveguide is projected directly onto the activesurface of a MEMS mirror. The system is designed, according to anembodiment of the invention, to be implemented on a silicon-on-insulator(SiO2) substrate because it is a widely available and mature andflexible technology and it is easier to merge with MEMS fabricationprocesses.

However, unlike the majority of MEMS configurations where the MEMSdevice is used as switch between two positions in this device the stateof the MEMS is important in all transition positions. Thecharacteristics of the MEMS thereby determine to a large degree thecharacteristics of the whole optical delay line system and by that theOCT system. The role of the MEMS is to alter the paths of the differentwavelengths in order to generate a new path difference between thewavelengths thereby creating the delay time. The inventors haveexploited two different MEMS and the characteristics of each one arepresented in schematic 300D in FIG. 3B which shows a schematicrepresentation of the integrated opto-electromechanical system. Theentire system fits in an area of 12 mm by 8 mm. The light from thebroadband source is coupled to the planar waveguide 350B that forms thedelay line with a ridge waveguide 350A. The beam then propagates intothe planar waveguide 350B where in it reflects off all the gratings andmirrors encountered.

First and second echelle gratings 355 and 390 respectively provide therequired wavelength dispersion such that the incident optical signal tothe device is split into several paths according to wavelength, as shownby the different lines in FIG. 3B. Within an embodiment of the inventionthe order number, in this case p=1.15 μm and m=1, and α=38 and β=16 arethe incidence and reflection angles, respectively. The first to thirdmirrors 360, 365, and 395 respectively are used to enlarge the pathdifference taken by the different wavelengths, that path differencebetween different wavelengths generates the time delay. The first tothird mirrors 360, 365, and 395 respectively are curved to allow therefocusing of the optical signals, and to prevent the beams from leavingthe system even with different tilt angles of the MEMS. The fourthmirror 385 is perpendicular to the incident optical signals toretro-reflect them such that they re-traverse the optical path back tothe ridge waveguide 350A. All these reflecting surfaces, first to fourthmirrors 360, 365, 395, and 385 are obtained by a simple etch step of theplanar waveguide. The other reflecting element is the MEMS 380 whichprovides for tunability of the delay induced by the device. Assumingthat the refractive index of silicon is 3.47 and the incidence angles onall reflecting surfaces are larger than the critical angle of 16.75°,the conditions for total internal reflections at these surfaces arefulfilled. Therefore, metallization of the reflecting surfaces is notneeded for first to third mirrors 360, 365, and 395 thereby simplifyingthe fabrication process.

Moreover, to avoid losses from clipping the optical signal by havingreflecting surfaces smaller than the optical beam, all optical surfacesin the device were designed to be at least 3 times larger than theincident beam radius, which is defined as where the power is reduced to1/e² from the peak. This ensures that the system has negligible clippinglosses. The MEMS Bragg mirror 380 within an embodiment of the inventionconsists of 5 and ½ pairs of silicon/air interfaces, with a 7.8 μmthickness, 300 μm long, and 12.46 μm wide as depicted in side view 300Cin FIG. 3A of the planar waveguide 350B and the MEMS mirror 380 ispresented. The MEMS Bragg mirror 380 is released from the substrate byremoving a 2 μm thick SiO₂ layer under the MEMS Bragg mirror 380,leaving a fixed anchor that is connected to an immovable part of thesubstrate. Graph 300B in FIG. 3A shows a simulation the reflection of Sand P polarization on the MEMS Bragg mirror 380; the MEMS Bragg mirror380 was optimized for the appropriate incidence angle in this case of54°. MEMS schematic 300E in FIG. 3B depicts a top view of the MEMS Braggmirror 380 and its comb drive with the anchor as the center of therotation. The largest losses within the device are sustained at the airgap between the MEMS Bragg mirror 380 and the planar waveguide 350B dueto the near field diffraction from the optical waveguide into freespace. In the embodiments of the invention implemented by the inventorsdue to the thickness of the planar waveguide 350B the optical mode, isrelatively large, 7.8 μm, and the air gap is small. The air gap isdependent upon the MEMS angle and varies between approximately 1 μm toapproximately 23 μm. Moreover, reflection at the planar waveguidesurface at this interface is suppressed with parylene antireflectioncoatings, the impact of this diffraction is minimal. For the fundamentalmode, the coupling between the reflected beam and the planar waveguideis always exceeding 83%.

Now referring to second schematic 400C the design outlined here isessentially the same as that depicted in schematic 300D in FIG. 3Bexcept that MEMS Bragg mirror 380 has been replaced by semi-circularMEMS mirror (SC-MEMSM) 480B. SC-MEMSM schematic 400A in FIG. 4 depicts atop view of the SC-MEMSM 480B and its deformation as grayscale shadingwhile in rotation. The SC-MEMSM anchor and its comb drive are not shownfor clarity. The SC-MEMSM has a half disk shape with thickness 7.8 μmand 300 μm radius which is released from the substrate by removing a 2μm thick SiO₂ layer under the half disk, leaving a fixed anchor that isconnected to an immovable part of the substrate.

This mirror shape keeps the air gap distance between the mirror and theplanar waveguide fixed during the rotation of the MEMS mirror, thuskeeping optical losses low and constant. This is important in order forthe losses to be as uniform as possible for all delay set points. Thelargest losses are sustained at the air gap because of the near fielddiffraction from the optical waveguide into free space. In this system,because the thickness of the waveguide and hence the optical mode, isrelatively large 7.8 μm, and the air gap is kept small in comparisonwith the optical mode, the impact of this diffraction is minimal. Forthe fundamental mode and a 0.98 μm air gap, the coupling between thediffracted beam and the planar waveguide is approximately 99%. Moreover,reflections are suppressed through the use of parylene antireflectioncoatings and by making the length of the air gap an odd multiple of onequarter of the broadband source central wavelength. This minimizesunwanted reflections through destructive interference.

Due to the architecture of the optical delay line circuit, secondschematic 400C, actuation of the SC-MEMSM is required in only oneangular direction, thus simplifying the actuator required, and reducingits impact on the resonant frequency on the SC-MEMSM. In addition, thecomb drive has angled stator fingers, in order to ensure that the combcan sufficiently rotate without its movable fingers colliding with thestator fingers. The SC-MEMSM must provide a rotational displacement θ,e.g. 2° degrees. The required vertical displacement, d, of the combdrive is geometrically defined by Equation (1)d=A _(M) tan(θ)  (1)A_(M) is the distance between the comb drive attachment and the mirrorcenter point, and θ is the rotational angle.

Optimal dimensions and placement for the comb drive were derived fromanalysis and simulation. Notably, fixing the maximal rotation to be 2°,end attaching the comb drive at 17 μm from the mirror center point therequired vertical displacement of the comb drive is calculated to beless than 0.6 μm. This displacement is achieved with a minimum comb gapof 1.8 μm and 24 150 μm-long by 17 μm-wide comb fingers.

Graph 300F in FIG. 3B depicts a plot of the induced optical delay versusrotation angle, where the time delay for an angle of 0° is taken as thereference. The calculations are given for rotation angles varying from−2°≤θ≤2°, which were achieved with the implemented SC-MEMSM devicesusing the fabrication processes and designs implemented. The time delaydifference achievable, −6.9 ps≤τ≤4.2 ps enables a total time delay rangelarger than 10 ps. The OCT time delay devices depicted in schematics300D in FIG. 3B and 400C in FIG. 4A whilst providing monolithic opticaltime delay lines for TD-OCT systems have two issues, the fabricationprocess and the linearity of delay with angle. Considering the formerthen the mirrors used in this design have cylindrical shape with aradius of curvature of 14 mm. If we consider that, the optical beam hasa beam-waist of approximately 50 μm, then the depth of the curved mirroris about 22 nm. This has a significant impact on the fabrication processdue to the limitations imposed by micro-fabrication technique on theachievable resolution of the mirror surface. This order of resolution isnot possible to implement with most of the lower cost and/or lowcomplexity manufacturing techniques. A change in the design and thus inthe size of the curved mirror is important therefore in considering asuccessful large-scale production of the optical delay line system.

The latter problem is the curvature to the delay, which is attributed tosmall group velocity dispersion (second order dispersion) within theoptical circuit. In high performance OCT systems where second orderdispersion could limit the resolution, this effect could be mitigatedwith a more complex echelle grating design, in which the grating periodis varied. In order to address these two issues the inventorsestablished new device designs and their respective time delay profilescalculated.

Within these designs the calculation were made for a grating period of 5μm, in silicon 1.44 μm, and the third grating order. The use of higherorder dispersion is beneficial because it generates a larger dispersionangle, which affects positively the total path difference and thusallows the creation of longer delay difference. First schematic 400B inFIG. 4A shows the representation of a first design of the secondgeneration time delay circuits. The entrance and the exit of thebroadband light are the same as used in the previous design except thefact; the angle of incidence is adjusted so that the reflected light ofthe central wavelength is normal to the gratings. The system consists of10 reflective surfaces. Two of these are first and second gratings 455Aand 490A respectively. In this case the angle of incidence of the firstgrating 455A is chosen in a way that the reflected is normal to thesurface of the grating whereas the angle of incidence on the secondgrating 490A is normal to the grating. The reason for this being toinsure the symmetrical dispersion on both sides of the centralwavelength.

First to fourth curved mirrors 460A, 465A, 470B and 475B act as thelenses of a bulk optical system and re-collimate the light inside thesystem. First to third flat mirrors 470A, 475A, and 485A provide foldingof the structure for smaller footprint. The SC-MEMSM 480A comprises thefinal reflective mirror and is placed such that at this surface theoptical beam has been re-focussed/re-collimated to occupy a small beamsize, in this case not more than 200 μm. The radius of the curvedmirrors is the same for first to fourth curved mirrors 460A, 465A, 470Band 475B. In fact the radius of these mirrors determines how large thesystem will be and the distance between the reflective mirrors definesthe difference in the path and hence the delay. As examples twodifferent setup are given and the delay time calculated for twodifferent radius of curvature of the mirrors.

First graph 400D in FIG. 4B the plot of the delay versus rotation anglewith the time delay for an angle of 0° taken as the reference for amirror curvature radius R=1.725 mm and the first grating with curvatureradius R=1.5 mm. These relatively small radiuses enable the use oflarge-scale fabrication technique commonly used in fabrication processesfor silicon MEMS devices. If we consider a beam radius of 50 μm, thedepth of the mirror necessary to cover this surface is 724 nm. Thisvalue is large enough to be considered for micro-fabrication. Forrotation angles −2°≤θ≤2°, the time delay difference varies from −3.15ps≤τ≤3.15 ps. This design allows 6.3 ps delay difference. The dependenceof the delay on the rotation angle is highly linear, this linearitymeans that the second order dispersion (group velocity dispersion) hasbeen reduced. The second radius size is R=0.69 mm this produces a mirrordepth of 1824 nm for a beam radius of 50 μm. This value is even largerthan the one from the last design and even fabrication techniques withlower resolution can be also considered. Second graph 400E in FIG. 4Bdepicts the delay versus rotation angle is plotted. The calculationsreveal that for a rotation angles varying from −2°≤θ≤2°. The time delaydifference varies from −1.4 ps≤τ≤1.4 ps and hence a total delay time of2.8 ps is obtained.

Subsequently additional design changes were made to yield a thirdgeneration optical time delay device, depicted in first schematic 500Ain FIG. 5. One such design change to facilitate the fabrication processis the position of the second grating 520. The entrance and the exitwere carefully chosen at 0.5 mm from the first grating 510. The curvedmirrors have a 0.966 mm radius of curvature and the incidence on thesecond grating 520 is again adjusted so that the reflected light isnormal to grating. The SC-MEMSM again has at this surface the lightre-collimated to occupy a very small surface. First curve 540 in graph500C in FIG. 5 shows the plot of the optical delay versus the rotationangle. Now for rotation angles varying from −2°≤θ≤2° the time delaydifference varies from −1.8 ps≤τ≤1.8 ps for a total delay time of 3.6 ps

Another design option is to implement an asymmetric design such asdepicted in second schematic 500B in FIG. 5 where the aim is to generatea maximum time delay difference, however this can generate a secondorder dispersion. As evident from the second schematic 500B this designis quite similar to the designs presented supra with some criticalchange. First the SC-MEMSM is placed earlier within the optical circuitallowing for increased propagation path for the spectrally dispersedbeam. This design is called asymmetric because the beam is notsymmetrically dispersed on both sides of the central wavelength and itis this fact that creates the group velocity dispersion. Accordingly,for −2°≤θ≤2° rotation angles, which are achievable with the proposedMEMS mirror, the time delay difference for the implemented circuitdesign as depicted by second curve 550 in FIG. 5 varies from −2.0ps≤τ≤6.4 ps yielding a maximum delay variation of 8.4 ps. Whilst largerthan the delays induced within the other circuits of comparabledimensions it is clearly a non-linear delay versus rotation angleprofile such that a small increase in controller complexity would berequired to provide the appropriate rotation for a desire delay.

The natural frequency of the SC-MEMSM devices simulated and implementedaccording to embodiments of the invention have a natural frequency thatexceed 12 kHz. Accordingly, the delay can be scanned at frequencies upto approximately 10 kHz

1C: Optical Spectrometer

Within many fields from research to quality control to safety opticalspectrometry is employed to determine and/or monitor materials eitherthrough the light reflected, transmitted, or radiated such as throughphotoluminscence for example. Different Materials have different opticalspectra for each of these and accordingly either a composition may bedetermined or the presence of a material confirmed by one or more ofthese optical spectra. For example, carbon monoxide has absorption linesat approximately 1.6 μm, 2.4 μm, and 4.8 μm whilst methane has broaderabsorption peaks at approximately 1.7 μm, 2.3 μm, 3.2 μm and 7.9 μm andammonia peaks at approximately 2 μm, 2.3 μm, 3 μm, 6 μm and 10 μm. Inanalytical systems the methodology is usually to scan across a range offrequencies to detect the absorption bands and then fit materials to theresulting spectra. In detection/alarm type applications the material ofinterest is known and hence the spectrometer needs to verify whetherthere is absorption or not. Accordingly, a spectrometer addressing thelater application may be required to only monitor a few wavelengths.

Referring to FIG. 6 there are depicted first and second opticalspectrometers 600A and 600B according to embodiments of the inventionproviding the ability to form compact, monolithic implementations withthe potential for low cost and small footprint. Such opticalspectrometers may scan rapidly across a predetermined range or stepquickly through a predetermined sequence. First spectrometer 600Acomprises an input optical waveguide 6250, which may alternatively be anoptical port for direct coupling from an external environment ratherthan via an optical fiber, which couples to planar waveguide 6200. Theoptical signals reflect from grating 6100 and are coupled to theSC-MEMSM 6150 wherein they are reflected towards the output waveguide6300. Disposed either side of the output waveguide 6300 are absorbers6350 to absorb the optical signals not coupled to the output waveguide6300 which may otherwise reflect through varying paths in varyingintensities to couple to the output waveguide 6300. Accordingly,rotation of the SC-MEMS 6150 results in different wavelengths beingcoupled to the output waveguide 6300. The output waveguide 6300 may becoupled to an optical photodetector within the visible and near-infrared(near-IR) or coupled to a micro-bolometer for the mid-IR.

Similarly, second spectrometer 600B comprises an input optical waveguide6700 that couples to a planar waveguide 6600 and subsequently to agrating 6750 and output waveguide 6900 disposed between absorbers 6950.However, in this instance the optical path from the input opticalwaveguide 6700 to the grating 6750 is folded by first reflector 6500 andthe optical path from the grating 6750 to the output waveguide 6900 issimilarly folded through second reflector 6550 and steered throughSC-MEMSM 6800. The grating 6750 is similarly a semi-circular MEMS devicebut now rather than a mirror on the back surface there is etched agrating. Accordingly, the angle of the grating 6750 to the optical beamcan be adjusted as well as the focusing optical signals may be steeredby the SC-MEMSM 6800. Accordingly, the second spectrometer 6800 mayprovide increased resolution through the folded optical path androtatable grating.

Within other embodiments of the invention a reflective filter structuresuch as described supra in respect of fourth image 100D in FIG. 4 andFIG. 2 may be employed to step through predetermined wavelengthsequence, defined by the reflective filters, wherein the reflectedsignal is coupled via a circulator to a photodetector or bolometer.

2. Designs for Different Optical Waveguide Technologies

2A: 850 NM and Mid-Infrared—Silicon Carbide Core

The choice of the wavelength of operation for an OCT system is acompromise between resolution and penetration depth. Scattering tissuesare usually imaged at 1.3 μm whereas in ophthalmic applications, 0.8 μmis usually preferred to resolve the details of the retina, see forexample Drexler et al in “Optical Coherence Tomography: Technology andApplications” (Springer, 2008). However, it would be beneficial forbroad exploitation of the integrated optical time delay circuit for thisto operate in these two wavelength ranges with minimal adjustments.However, silicon waveguides are not transparent below 1.1 μm.Furthermore, for other applications such as molecular spectroscopy, forexample, it would be desirable operate in the mid-infrared (mid-IR)between 3.0 μm≤λ≤5.0 μm Stoichiometric amorphous silicon nitride istransparent 0.3 μm≤λ≤11.0 μm whilst hexagonal crystalline siliconcarbide transmits light 0.5 μm≤λ≤20.0 μm, see for example Palik in“Handbook of Optical Constants of Solids” (Academic Press, 1985). Bothmaterials can be deposited through a variety of processes, which makesit easy and affordable to tailor waveguides to multiple applications.Whilst deposited materials can have optical properties that deviate fromthose reported for bulk materials the experimental measurements found inthe literature indicate that for each of these materials these haveacceptable properties for low loss optical waveguides over the desiredtransparency window.

Accordingly, the inventors propose a novel integrated waveguidestructure supporting MEMS manufacturing, depicted in first waveguidecross-section 700A, where the core is silicon carbide 770 and thecladding layers are formed with silicon nitride 740. With the ability todeposit silicon carbide 770 in multiple stages an alternative design mayimplement MEMS elements such as MEMS mirror entirely from siliconcarbide 770 in conjunction with silicon carbide optical waveguides.Other ceramic materials in addition to silicon carbide and siliconnitride that may be employed according to the devices being implementinclude silicon dioxide (SiO₂), aluminum nitride (AlN), alumina (Al₂O₃),zirconia (ZrO₂), and diamond (C).

2B: Telecommunications Window (1300 nm & 1550 nm)—Silicon Nitride Core

2B.1: Optical Waveguide Design

Referring to third waveguide cross-section 700B in FIG. 7 there isdepicted a waveguide geometry according to an embodiment of theinvention comprising a 5 μm lower silicon dioxide 730 cladding, a 70 nmsilicon nitride (Si₃N₄) 740 core, and a 5 μm upper silicon dioxide 730cladding. The waveguide cross-section 700B is depicted where the opticalwaveguide couples via the air gap to the SC-MEMSM which is similarlyformed from the same material stack as the optical waveguide.

Referring to Table 1 there is depicted the calculated coupling forvarying air gap with varying silicon nitride 740 core thickness. It isevident from this analysis that thinning the silicon nitride 740 coreresults in an increasing optical beam waist, increased coupling at aninitial air gap of 200 nm, and increased air gap for a predeterminedoptical insertion loss limit, e.g. a 1 dB insertion loss penalty (80%).Accordingly, for an effective waveguide mode index of 1.492 the idealanti-reflection coating for the optical waveguide to air would have arefractive index of 1.23

TABLE 1 Coupling Efficiency with Air Gap Length for Varying WaveguideGeometries Si₃N₄ Beam Thickness Effective Waist Air Gap Length (nm)Index (nm) 200 nm 500 nm 750 nm 1 μm 2 μm 3 μm 570 1.829 394 69.56%20.71% 200 1.600 441 78.98% 33.00% 15.91% 100 1.512 720 96.48% 81.12%64.885 49.48% 16.22% 70 1.492 986 98.98% 93.92% 87.20% 79.12% 46.72%25.70%

2B.2: MEMS Circuit Designs

Referring to FIG. 8 there are depicted first to third circuits 800A,800C, and 800E respectively for SC-MEMSM with design radii of 0.5 mm,0.75 mm, and 1.00 mm. In each instance the optical waveguides couplingto the Bragg reflectors are spaced 200 μm away from the edge of theSC-MEMSM and in each instance the distance from the pivot mounting ofthe SC-MEMSM to the optical waveguides is equal to the radius of theSC-MEMSM. Accordingly, the resulting widths of the SC-MEMSM in the threedesigns depicted in first to third circuits 800A to 800C respectivelyare 500 μm, 750 μm, and 950 μm. Accordingly considering a maximumangular rotation of the SC-MEMSM as ±3° then the lateral spacing betweenthe upper and lower end waveguides are 52 μm, 78 μm, and 105 μmrespectively. Referring to first to third graphs 800B, 800D, and 800Frespectively there are therefore depicted the number of accessiblechannels for optical waveguides having spacings of 0.5 μm and 0.75 μmrespectively. Accordingly, for 0.75 μm spaced waveguides the maximumnumber of channels accessible are 36 (±18 channels from centre), 54 (±27channels from centre), and 74 (±37 channels from centre) at design radiiof 0.5 mm, 0.75 mm, and 1.00 mm. The corresponding maximum numbers ofchannels accessible for these design radii with 0.5 μm channel spacingare 40, 60, and 80 respectively.

Once the optical signals have been coupled by the SC-MEMSM into theoptical waveguides connecting to the Bragg gratings then the waveguidespacing should be increased in order to reduce the optical (parasitic)coupling from the desired waveguide to the adjacent waveguides.Referring to first and second graphs 900A and 900B in FIG. 9A thecalculated coupling coefficient for 70 nm and 100 nm thick siliconnitride 720 cores are depicted as a function of waveguide gap (spacing).In first graph 900A the plots represent waveguide widths of 1.8 μm and4.0 μm respectively, the single mode width limit being approximately 4.5μm. In second graph the plot represent waveguide widths of 1.0 μm and2.8 μm respectively, the single mode width limit being approximately 3.0μm. Accordingly, the resulting waveguide gaps for 20 dB power cross-talkbetween the target waveguide and adjacent waveguides are given in Table2 for a parallel waveguide region of length 10 mm

TABLE 2 Waveguide Spacings for Bragg Waveguide Sections with 70 nmSilicon Nitride Core 70 nm Thickness 100 nm Thickness W (μm) Gap (μm) W(μm) Gap (μm) 1.8 8.9 2.8 5.0 4.0 5.9 1.0 9.7

Within the simulations of all waveguides a commercial Institute ofMicroelectronics (IME) process exploiting deep UV stepper basedphotolithography at 248 nm was assumed. This offers 180 nm and 200 nmminimum exclusion distances. Accordingly, Bragg gratings were modelledin both the 70 nm and 100 nm thick silicon nitride 720 cores toestablish the bandwidth (Δλ) which is the wavelength spacing between thefirst minima of the grating transfer function which is given by Equation(2).

$\begin{matrix}{{\Delta\lambda} = {\left\lbrack \frac{2\delta\; n_{0}\eta}{\pi} \right\rbrack\lambda}} & (2)\end{matrix}$where δn₀ is the variation in refractive index between the refractiveindex of the waveguide with and without the grating, λ the centrewavelength, and η the fraction of the power within the core of thewaveguide. Accordingly, the resulting grating length required as afunction of Δλ for varying waveguide reflectivity values are depictedfor these 70 nm and 100 nm thick silicon nitride 740 cores in first andsecond graphs 900C and 900D respectively in FIG. 9B. In each instancefor a Δλ=0.15 the grating length must be greater than 10 mm Referring toTables 3 and 4 there are depicted grating design and grating simulationresults for cladding modulated first order gratings within each of the70 nm and 100 nm thick silicon nitride 720 cores respectively where Λ isthe pitch of the grating, g the separation of the inner edges ofwaveguide and grating, and w is the width of the grating elements.

TABLE 3 Grating Design Parameters for Nitride Core Waveguide DesignsThickness (nm) N_(P) Λ (nm) g (μm) w (nm) 70 8322 533 1.000 180 100 8123527 1.050 180

TABLE 4 Grating Assumptions and Simulations for Nitride Core WaveguideDesigns Theoretical Assumptions Simulation Results Δλ Δλ (null to L(null to Δλ L Thickness null) R (mm) K null) (3 dB) R (mm) 70 0.4 0.84.436 325 0.41 0.25 0.82 4.4 100 0.4 0.8 4.265 338 0.39 0.24 0.81 4.3

2B.3: MEMS Process Flow

Referring to first schematic 1000A in FIG. 10 there is depicted a planview of a wavelength dependent reflector (WADER) circuit comprising anSC-MEMSM mirror 1020 attached to a comb drive 1010 and then a Braggreflector array 1040 comprising a central channel waveguide 1050 whichcouples light into and out of the wavelength dependent reflector circuitand arrays of Bragg waveguides 1060 disposed either side of the channelwaveguide via the air gap 1030 and the SC-MEMSM 1020. In otherembodiments of the wavelength dependent reflector circuit the Braggwaveguides 1060 may be disposed symmetrically either side of the channelwaveguide, asymmetrically with different channel counts either side ofthe channel waveguide, and asymmetrically to one side of the channelwaveguide. Such design considerations may be based upon factorsincluding, but not limited to, the angular rotation range of theSC-MEMSM mirror 1020, the number of wavelength channels, the design ofthe MEMS comb drive 1010, position of other optical elements such asoptical gain elements, photodetectors, etc., and the design of theelectrostatic driver circuit for the MEMS comb drive 1010.

Accordingly, referring to second schematic 1000B in FIG. 10 across-section of the WADER circuit is depicted in cross-sectional viewcomprising silicon dioxide (SiO2) 730, silicon (Si) 720, and aluminum(Al) 710 which has already been patterned and etched. Considering atypical silicon-on-insulator (SOI) substrate then the Si 720 is 5 μmthick. The Al 710 may be sputtered with a thickness of 300 nm whichafter patterning through a lithography process may be removed using astandard Al wet etch process. Subsequently in third schematic 1000C theWADER circuit is depicted after the exposed Si 720 has been patteredlithographically and deep etched to remove 4.5 μm using a deep reactiveion etching (DRIE) process using sulphur hexafluoride (SF₆) andoctafluorocyclobutane (C₄F₈) after which the resist is stripped.

Now referring to fourth schematic 1000D in FIG. 11 the optical waveguidelayer stack is deposited comprising 4 μm SiO2 730, 100 nm siliconnitride (Si3N4) 740, and 4 μm SiO2 730. The deposition being for examplethrough chemical vapour deposition (CVD). Next in fifth schematic 1000Ein FIG. 11 the WADER circuit is depicted after the optical waveguidesand comb drive openings have been defined, using a DRIE etching processwith a SF₆—C₄F₈—Argon (Ar) process having an aspect ratio of 1:1.6 toetch the 4 μm SiO2 730—100 nm silicon nitride (Si3N4) 740—4 μm SiO2 730stack, and the comb drive has been defined using a DRIE etching processwith a SF₆—C₄F₈ process having an aspect ratio of 1:1 to etch the 5 μmSi 720.

Subsequently in sixth schematic 1000F in FIG. 11 the WADER circuitcross-section is depicted after the air gap has been formed and theexcess regions of the optical waveguides atop the comb drive etc. havebeen removed. These steps are achieved using a DRIE etching process witha SF₆—C₄F₈—Argon (Ar) process having an aspect ratio of 1:8 to etch theSiO2 730—Si3N4 740—SiO2 730 stack, and the comb drive has been definedusing a DRIE etching process with a SF₆—C₄F₈ having an aspect ratio of2:1 to etch the 0.5 μm Si 720. Whereas the preceding steps were carriedout with a critical dimension of approximately 5 μm the photolithographyfor the air gap processes have a critical dimension of approximately 1μm. Within a variant of the process flow the process sequences leadingto fifth and sixth schematics 1000E and 1000F may be reversed such thatthe excess waveguide stack is removed and the waveguides define prior toetching the actuator.

Next in seventh schematic 1000G in FIG. 12 the Bragg grating sections ofthe optical Bragg reflectors are photolithographically defined andetched using a DRIE etching process with a SF₆—C₄F₈—Argon (Ar) processhaving an aspect ratio of 1:8 to partially etch the upper claddingcomprising SiO2 730. Due to the requirements of the Bragg gratingprocess resolution of approximately 180 nm or better is necessary atthis stage.

Now referring to eighth schematic 1000H in FIG. 12 a reflective layer,gold (Au) 780, is deposited and patterned onto the SC-MEMSM mirrorsidewalls and anti-reflection (AR) coatings 760 are deposited andpatterned onto the SC-MEMSM mirror sidewall and optical waveguidesidewall either side of the air gap. The AR coating 760 may be magnesiumfluoride, MgF₂, for example with a thickness of 280 nm. Subsequently asdepicted in ninth schematic 1000I the front surface of the WADER circuitis protected for wafer backside processing steps that follow.Accordingly, polyimide 750 with a thickness of 5 μm may be spin-coatedonto the wafer and cured, e.g. 300° C. for 2 hours. Optionally at thispoint the substrate may also be thinned using Chemical MechanicalPolishing (CMP) for example.

In ninth schematic 1000I in FIG. 13 the substrate, e.g. silicon, islithographically processed to define the trench below the MEMS combdrive and SC-MEMSM mirror sections of the WADER circuit. This may, forexample, be via a DRIE using SF₆—C₄F₈ stopping at the SiO2 730 layer.Then in tenth schematic 1000J the SiO2 720 is etched from the backsideusing an RIE process, for example, followed by resist stripping, waferdicing, polyimide removal by plasma ashing, for example, and mechanicalpolishing of the WADER circuit die sidewall for connection between thechannel waveguide and optical fiber. The final device being depicted ineleventh schematic 1000K.

2C. Telecommunications Window (1300 nm & 1550 nm)—Silicon Core

2C.1 Optical Waveguide Design

Referring to second waveguide cross-section 700C in FIG. 7 there isdepicted a waveguide geometry according to an embodiment of theinvention comprising a lower silicon dioxide 730 lower cladding, asilicon 720 core, and air upper cladding. The waveguide cross-section700C is depicted where the optical waveguide couples via the air gap tothe SC-MEMSM which is similarly formed from the same material stack asthe optical waveguide.

However, the thickness limit of the silicon (Si) for a single-modewaveguide is 220 nm which is too thin for MEMS devices. However, at athickness of 1 μm 5 modes exist within the silicon having modal indicesof n=3.405, 3.203, 2.845, 2.281, 1.487 and accordingly a rib waveguidegeometry is employed in order to select the fundamental mode. Due to therefractive indices the anti-reflection (AR) layer on the air gap of theoptical waveguide and SC-MEMSM can be formed from parylene with arefractive index of 1.66. The thickness of the AR coating would be 233nm.

Referring to Table 5 there is depicted the calculated coupling forvarying air gap with varying silicon 720 core thickness. It is evidentfrom this analysis that thinning the silicon 720 core results in adecreasing optical beam waist and decreasing coupling at an initial airgap of 200 nm. Accordingly, an increased thickness is preferred for apredetermined optical insertion loss limit, e.g. a 1 dB insertion losspenalty (80%).

TABLE 5 Coupling Efficiency with Air Gap Length for Varying WaveguideGeometries Si Beam Thickness Effective Waist Air Gap Length (nm) Index(nm) 200 nm 500 nm 750 nm 1 μm 2 μm 3 μm 10,000 3.469 3,860 99.02 5,0003.466 1,960 99.94 99.58 99.08 98.37 93.63 86.27 3,000 3.461 1,200 99.5497.12 88.99 77.15 63.73 39.41 2,000 3.451 820 97.87 87.54 74.47 60.1219.93 1,500 3.438 631 94.06 69.06 45.97 28.64 1,000 3.404 440 77.8627.86 500 3.262 252 19.22 220 2.820 162

As depicted in FIG. 14 in first image 1410 a cross-section of thesilicon-on-insulator (SOI) rib waveguide is depicted comprising SiO2 730lower cladding (the insulator) and Si 720 having an original thickness,H, that has been etched to form a rib wherein the remaining Si 720adjacent the rib is a slab of height r.H. The rib waveguide being airclad. Accordingly, referring to Table 6 below the waveguide geometriesfor maximum confinement as a function of rib waveguide height, H arepresented. An example of the mode structure for a rib waveguide employedis depicted in second schematic 1420.

TABLE 6 Single Mode Rib Waveguides for Maximum Confinement with VaryingRib Height Height (H) 3.40 μm 2.00 μm 1.00 μm Slab (r.H) 2.04 μm 1.20 μm0.51 μm Width (W) 2.17 μm 1.40 μm 0.52 μm Fundamental Mode Width 3.00 μm2.50 μm 2.00 μm

Because of the slab waveguide there can be significant leakage(cross-coupling) between the rib waveguides if they are placed too closeto one another. Accordingly, this imposes a lower limit on theirseparation thereby reducing the number of channels within the devicesaccording to these embodiments of the invention. This is depicted inthird and fourth images 1430 and 1440 respectively for a rib waveguidearray wherein light is coupled into the central waveguide in third image1430 and to an adjacent pair of waveguides in fourth image 1440. In eachinstance, power coupling is evident between the adjacent waveguides.Accordingly, in contrast to the separations of 0.50 μm and 0.75 μmwithin the silicon nitride design analysis in Section 2B supra theseparations within the design analysis for the SOI rib waveguides were4.50 μm and 5.5 μm respectively.

2C.2: MEMS Circuit Designs

Referring to FIG. 15 there are depicted first to third circuits 1500A,1500C, and 1500E respectively for SC-MEMSM with design radii of 0.5 mm,0.75 mm, and 2.00 mm respectively. In each instance the opticalwaveguides coupling to the Bragg reflectors are spaced 200 μm away fromthe edge of the SC-MEMSM and in each instance the distance from thepivot mounting of the SC-MEMSM to the optical waveguides is equal to theradius of the SC-MEMSM. Accordingly, the resulting widths of theSC-MEMSM in the three designs depicted in first to third circuits 1500Ato 1500C respectively are 160 μm, 250 μm, and 680 μm. Accordinglyconsidering a maximum angular rotation of the SC-MEMSM as ±3° then thelateral spacing between the upper and lower end waveguides are 52 μm, 78μm, and 209 μm respectively. Referring to first to third graphs 1500B,1500D, and 1500F respectively there are therefore depicted the number ofaccessible channels for optical waveguides having spacings of 4.50 μmand 5.5 μm respectively. Accordingly, for 5.5 μm spaced waveguides themaximum number of channels accessible are 16 (±8 channels from centre),26 (±13 channels from centre), and 74 (±37 channels from centre) atdesign radii of 0.5 mm, 0.75 mm, and 2.00 mm respectively. Thecorresponding maximum numbers of channels accessible for these designradii with 4.50 μm channel spacing are 20, 32, and 90 respectively.

Now referring to FIG. 16 there are depicted first to third graphs 1600Ato 1600C respectively depicting the grating length required as afunction of Δλ for varying waveguide reflectivity values for 3.40 μm,2.00 μm and 1.00 μm rib height silicon 720 waveguides. In each instancefor a Δλ=0.15 the grating length must be greater than 6 mm. Referring toTables 7A to 9B there are presented grating assumptions and gratingsimulation results for cladding modulated first order gratings withineach of the 3.40 μm, 2.00 μm and 1.00 μm rib height silicon 720waveguides respectively where A is the pitch of the grating, g theseparation of the inner edges of waveguide and grating, and w is thewidth of the grating elements.

TABLE 7A Grating Design Parameters for 3.40 μm Silicon Rib WaveguideDesigns Grating Type N_(P) Λ (nm) g (nm) w (nm) A 21807 223 800 180 B7258 670 200 180 C 7258 670 200 190

TABLE 7B Grating Assumptions and Simulations for 3.40 μm Silicon RibWaveguide Designs Theoretical Assumptions Simulation Results Δλ Δλ (nullto L (null to Δλ L Design null) R (mm) K null) (3 dB) R (mm) A 0.15 0.84.863 296 0.15 0.091 0.79 4.90 B 0.15 0.082 0.77 4.86 C 0.15 0.088 0.804.86

TABLE 8A Grating Design Parameters for 2.00 μm Silicon Rib WaveguideDesigns Grating Type N_(P) Λ (nm) g (nm) w (nm) A 16200 224 800 180 B21607 224 950 180 C 7202 673 500 190

TABLE 8B Grating Assumptions and Simulations for 2.00 μm Silicon RibWaveguide Designs Theoretical Assumptions Simulation Results Δλ Δλ (nullto L (null to Δλ L Design null) R (mm) K null) (3 dB) R (mm) A 0.20 0.84.863 296 0.15 0.091 0.79 4.90 B 0.15 4.84 298 0.15 0.082 0.77 4.86 C0.15 0.088 0.80 4.86

TABLE 9A Grating Design Parameters for 1.00 μm Silicon Rib WaveguideDesigns Grating Type N_(P) Λ (nm) g (nm) w (nm) A 20409 232 1000 180 B6803 696 750 180

TABLE 9B Grating Assumptions and Simulations for 1.00 μm Silicon RibWaveguide Designs Theoretical Assumptions Simulation Results Δλ Δλ (nullto L (null to Δλ L Design null) R (mm) K null) (3dB) R (mm) A 0.15 0.84.735 232 0.155 0.091 0.84 4.734 B 3.735 291 0.220 0.110 0.86 4.700

Accordingly, as depicted in FIG. 16 a design flow 1600D for the Braggreflectors is depicted wherein in step 1610 the desired Δλ andreflectance R are chosen such that in step 1620 the grating length L andgrating wave-vector K. Subsequently in step 1630 the design process iscompleted by finding a grating design that yields the desired gratingwave-vector K for the assumed waveguide.

2C.3: MEMS Process Flow

Referring to first schematic 1700A in FIG. 17 there is depicted a planview of a wavelength dependent reflector (WADER) circuit comprising anSC-MEMSM mirror 1020 attached to a comb drive 1010 and then a Braggreflector array 1040 comprising a central channel waveguide 1050 whichcouples light into and out of the wavelength dependent reflector circuitand arrays of Bragg waveguides 1060 disposed either side of the channelwaveguide. In other embodiments of the wavelength dependent reflectorcircuit the Bragg waveguides 1060 may be disposed symmetrically eitherside of the channel waveguide, asymmetrically with different channelcounts either side of the channel waveguide, and asymmetrically to oneside of the channel waveguide. Such design considerations may be basedupon factors including, but not limited to, the angular rotation rangeof the SC-MEMSM mirror 1020, the number of wavelength channels, thedesign of the MEMS comb drive 1010, and the design of the electrostaticdriver circuit for the MEMS comb drive 1010.

Accordingly, referring to second schematic 1700B in FIG. 17 across-section of the WADER circuit is depicted in cross-sectional viewcomprising silicon dioxide (SiO2) 730, silicon (Si) 720, and aluminum(Al) 710 which has already been patterned and etched in third schematic1700C. Considering a typical silicon-on-insulator (SOI) substrate thenthe Si 720 is 5 μm thick. The Al 710 may be sputtered with a thicknessof 300 nm which after patterning through a lithography process may beremoved using a standard Al wet etch process. Subsequently in fourthschematic 1700D in FIG. 18 the WADER circuit is depicted after theexposed Si 720 has been pattered lithographically and deep etched toremove to form the comb drive using a deep reactive ion etching (DRIE)process using sulphur hexafluoride (SF₆) and octafluorocyclobutane(C₄F₈) after which the resist is stripped. Next in fifth schematic 1700Ein FIG. 18 the WADER circuit is depicted after a 3 μm RIE to etch the Si720 to form the waveguide regions with a thickness of 2 μm

Subsequently in sixth schematic 1700F in FIG. 18 the WADER circuitcross-section is depicted after the air gap has been formed using a RIEetching process with SF₆ to etch the 2 μm Si 720 with an aspect ratio of1:2. Whereas the preceding steps were carried out with a criticaldimension of approximately 5 μm the photolithography for the next stepof Bragg grating fabrication is performed with electron-beam (e-beam)lithography or photolithography process with a critical dimension of 180nm. Accordingly, in seventh schematic 1700G in FIG. 19 the Bragg gratingsections of the optical Bragg reflectors are etched using an SF₆ RIEetching process with an aspect ratio of 1:4.5 to partially etch the Si720 by removing 800 nm.

Now referring to eighth schematic 1700H in FIG. 19 a reflective layer,gold (Au) 780, is deposited and patterned onto the SC-MEMSM mirrorsidewalls and anti-reflection (AR) coatings are deposited and patternedonto the SC-MEMSM mirror sidewall and optical waveguide sidewall eitherside of the air gap. The AR coating may be paralyne 790 for example asdescribed supra of thickness 233 nm. Plan schematic 1700I shows themirror sidewalls and air gap as the cross-section in eighth schematic1700H is through the SC-MEMSM mirror and its anchor and does notintersect the SC-MEMSM mirror sidewall.

Subsequently as depicted in ninth schematic 1700I the front surface ofthe WADER circuit is protected for wafer backside processing steps thatfollow. Accordingly, polyimide 750 with a thickness of 5 μm may bespin-coated onto the wafer and cured, e.g. 300° C. for 2 hours.Alternatively, photoresist or other materials may be employed to coatand protect the wafer prior to backside processing. Optionally at thispoint the substrate may also be thinned using Chemical MechanicalPolishing (CMP) for example.

In tenth schematic 1700K in FIG. 20 the substrate, e.g. silicon, islithographically processed to define the trench below the MEMS combdrive and SC-MEMSM mirror sections of the WADER circuit. This may, forexample, be via a DRIE using SF₆—C₄F₈ stopping at the SiO₂ 720 layer.Then in eleventh schematic 1700L the SiO₂ 720 is etched from thebackside using an RIE process, for example, followed by resiststripping, wafer dicing, polyimide removal by plasma ashing, forexample, and mechanical polishing of the WADER circuit die sidewall forconnection between the channel waveguide and optical fiber.

3. Semi-Circular MEMS Mirror (SC-MEMSM) Actuator Design

Referring to FIG. 21 there is depicted a semi-circular MEMS mirror(SC-MEMSM) design 2100 according to an embodiment of the inventionexploiting electrostatic actuation with slanted fingers. Accordingly asdesigned the SC-MEMSM will rotate when the 8 μm SC-MEMSM fingers areelectrostatically attracted to the drive contacts. The SC-MEMSM fingeradjacent the solid V_(DD) electrode is angled at 4.5° whilst the otherSC-MEMSM fingers adjacent V_(DD) electrode fingers are angled at 6°. Thedisc of the SC-MEMSM subtends an arc of 135° and is attached via a 3 μmpivot element to the V_(SS) electrode. However, it would be evident thatthe mirror shape attached to all the actuators presented herein could bereadily modified and/or have any subtending angle. Such design factorswill be determined by the optical circuit within which the SC-MEMSM isused. Towards the end of the solid V_(DD) electrode by the SC-MEMSMfinger a stopper electrode is provided which is selectively biased toV_(SS). Also depicted in FIG. 21 are first to third scanning electronmicroscope (SEM) images 2110, 2120, and 2150 respectively for thefabricated SC-MEMSM mirror.

Now referring to FIG. 22A there is depicted a semi-circular MEMS mirror(SC-MEMSM) design 2200 according to an embodiment of the inventionexploiting electrostatic actuation with a comb drive and slanted finger.Accordingly as designed the SC-MEMSM will rotate when the 11 μm SC-MEMSMfingers within the comb drive are electrostatically attracted to thedrive contacts. The SC-MEMSM also comprises a SC-MEMSM finger adjacentthe solid V_(DD) electrode is angled at 4.5° whilst the SC-MEMSM combdrive fingers that rotate are angled at 6° where these are attractedtowards the other comb drive fingers (right-hand side) and are notangled where these will be repelled away from the other comb drivefingers (left-hand side). The disc of the SC-MEMSM subtends an arc of135° and is attached via a 3 μm pivot element to the V_(SS) electrode.Towards the end of the solid V_(DD) electrode by the 8 μm SC-MEMSMfinger a stopper electrode is provided which is selectively biased toV_(SS). Also depicted in FIG. 22 are first to third scanning electronmicroscope (SEM) images 2210, 2220, and 2250 respectively for thefabricated SC-MEMSM mirror. Detail for the comb drive portion of theSC-MEMSM drive is depicted in FIG. 22B. Optionally, a second actuatorcould be added on the other side of the SC-MEMSM mirror to achieve apull-pull configuration allowing for a greater actuation angle.

Referring to FIG. 23 there is depicted another SC-MEMSM mirror accordingto an embodiment of the invention wherein the comb drive consists of aplurality of curved fingers biased at V_(SS) which are disposed betweenthe fingers of the electrostatic drive element biased at V_(DD).Optionally, a second electrostatic drive element may be added to theother side end of the SE-MEMSM curved fingers to add another pullingdirection and thus increase the actuation angle as shown with SC-MEMSMmirror 2350 in FIG. 23.

Referring to FIG. 24 there are depicted first to fourth graphs 2410 to2440 respectively for the SC-MEMSM mirror designs 2100, 2350, 2300, and2200 respectively. First to third graphs 2410 to 2430 being simulatedrotation angle versus voltage plots whereas fourth graph 2440 is ameasured rotation angle versus voltage plot. Accordingly these indicatethat the required voltage to rotate the SC-MEMSM 2° are approximately200V, 200V, and 320V respectively from simulations and 170V for theexperimental data. FIG. 25 depicts an optical micrograph 2500 of anSC-MEMSM of the design depicted in FIG. 21 biassed at 140V showing theSC-MEMSM rotated around from its unbiased state. Graph 2550 depicts themeasured rotation versus voltage wherein a 2° rotation is achieved at140V. The actuation voltages may be reduced by decreasing the gap sizewithin the respective actuator structures.

4. SC-MEMSM Mirror Design

Within the embodiments of the invention, process flows, and variantsdiscussed and described supra in respect of FIGS. 1 to 25 it would beevident to one skilled in the art that whilst SC-MEMSM designs featurethroughout that there are two different classes of SC-MEMSM within thesefigures that each share a semicircular disk rotating with a small airgap adjacent to a curved planar waveguide structure. However, the rearreflecting mirror surface of the SC-MEMSM differs in the two classes.

The first class is where the rear reflecting mirror surface is a planarmirror such that the optical signals impinging upon it at an angle β° tothe normal of the planar surface are reflect and propagate away at anangle β° on the other side of the normal. Such a rear reflecting planarmirror is depicted in FIGS. 4, 6, 10, 17, 21, 22A, 23, 25, and 26.Accordingly a collimated optical signal will reflect and maintaincollimation whilst a diverging beam with reflect and still diverge.

The second is a curved back mirror where the reflecting mirror surfacehas a predetermined profile such that the normal to the mirror surfacevaries across the surface and hence whilst locally each optical signalwill reflect according to the normal at its point of incidence theoverall effect of the mirror on beam is determined by the profile of themirror and the point at which the optical beam impinges. Considering therear reflecting planar mirror as depicted in FIGS. 1, 2, 8, 15 thesurface is concave with respect to the impinging optical beam which isaligned to the axis of the curved mirror surface. As a result theconcave rear reflecting mirror results in focusing of a diverging beamsuch that where the radius of curvature of the rear surface equals itsdistance from the source of a diverging optical beam it will refocus thebeam back at the same distance but laterally offset according to therotation angle of the SC-MEMSM. Hence, for example the WADER depicted inthird schematic 1500E in FIG. 15 has a radius of curvature for the rearreflecting surface of the SC-MEMSM of 2000 μm with the pivot point 2000μm from the central channel waveguide. The front surface of the SC-MEMSMis the curved surface of radius 300 μm

However, it would be evident that other profiles for the rear reflectingmirror surface may be employed according to the functionality of theoverall optical circuit and the characteristics of the mirror required.For example, the rear surface may be parabolic to focus an impingingcollimated beam or generate a collimated beam from an optical source. Inaddition, the surface could be corrugated in order to implement amovable dispersive element, e.g. an echelle grating.

As discussed supra with respect to the design and performance of theoptical circuits comprising SC-MEMSM elements the size of the gap canfactor significantly into this performance. In some optical circuits theSC-MEMSM mirror is employed once to set the device containing theoptical circuit, e.g. setting the wavelength of a wavelength tunabletransmitter at installation into the network. In others the SC-MEMSMmirror may be periodically or aperiodically set according to a resettingof the device rather than continuously scanned as occurs within the OCTdevice. In these instances the inventors have established a modifiedSC-MEMSM 2600 as depicted in FIG. 26 wherein the anchor 2640 for theSC-MEMSM mirror 2630 is moveable through the action of an electrostatictranslational (linear) actuator 2610 once rotated by SC-MEMSM drive2620. Accordingly, the electrostatic linear actuator 2610 may push theSC-MEMSM mirror 2630 towards the surface of the optical circuit after itis rotated through SC-MEMSM drive 2620. Once the rotation is completethe electrostatic linear actuator 2610 may be employed, for example, toclose the gap to a predetermined value, close the gap till the SC-MEMSMis in friction contact with the optical circuit, close the gap until theSC-MEMSM contacts short stoppers 2650 on the other side of the air gap,or close the gap to a predetermined value and apply a dithering to thegap such as described below in respect of Section 6. It would be evidentthat the translational actuator 2610 allows the gap dimension to bereduced significantly from the fabricated one, thereby reducing theoptical loss in the air gap as well as the cost of manufacturing andincreasing yields of the MEMS.

Now referring to FIG. 27 there is depicted another variant of anSC-MEMSM according to an embodiment of the invention employing arotational actuator 2740 controlling the rotation of a MEMS mirror 2750in combination with gap actuators 2710 to adjust the MEMSM gap andlatching actuator 2720/locking actuators 2730 to maintain MEMSM positiononce set. The MEMS rotational actuator 2740 comprises 8 arcuateelectrodes of width 12 μm with different radius to the ground electrodesdisposed at either end such that on one end the gaps are 10 μm/7 μm tothe outer/inner electrode and at the other end they are 7 μm/10 μm. Thearm between rotational actuator 2740 and MEMS mirror 2750 has a width of12 μm. The MEMS mirror 2750 itself has a rear surface (reflectingsurface) of radius 550 μm and a front surface radius of 300 μm.Referring to FIG. 31 there is depicted the simulated rotation angleversus electrostatic voltage for a MEMSM design such as depicted in FIG.27 indicating that a rotation angle of 5° can be achieved for 350Velectrostatic potential. An optical micrograph of the SC-MEMSM depictedin FIG. 27 is presented in FIG. 36.

5. MEMS Gap Actuator

As indicated within FIG. 27 the SC-MEMSM according to an embodiment ofthe invention employs a pair of gap actuators 2710 which are depicted inFIG. 28 in first image 2800A together with the MEMS mirror and facet2810 of the circuit of which the SC-MEMSM forms part. Second and thirdimages 2800B and 2800C in FIG. 28 depict optical micrographs of thestructure as fabricated. In operations the gap actuators 2710 allow thefront surface of the MEMS mirror 2750 to be moved relative to facet2810. Accordingly, the optical gap between the front surface of the MEMSmirror 2750 and facet 2810 can be adjusted and/or eliminated. Referringto FIG. 32 depicts the displacement versus electrostatic voltage for aMEMSM gap actuator 2710 is depicted indicated a movement ofapproximately 0.5 μm at an electrostatic potential of 100V.

The MEMSM gap actuators 2710 are intended to bring the MEMS mirror 2750closer to the fixed portion of the optical integrated circuit, e.g.facet 2810. This allows for a reduction in optical loss within the airgap. The minimum separation is defined by the fabrication process gridsize utilised to create stoppers and not the separation dictated by theminimal feature size of the process. This allows the inventors tosignificantly reduce the size of the air gap between the mirror and theinput and output waveguides when the gap is closed, and hence tominimize optical propagation losses. Within the exemplary embodimentdepicted in FIG. 28 the gap closer structure developed was implementedwith a fabrication process having a 3 μm minimum feature size consist oftwo springs of three sections of length 205 μm and width 3 μm attachingeach side of the mirror to the mast, namely the beam connecting the MEMSmirror 2750 to the rotational actuator. These springs allow a movementtowards the actuator that reduces the gap to a “fixed” distance managedby the “stoppers” on each side of the actuator. Those stoppers being 3μm “dimples” to prevent mirror adhesion to the actuator. The gap closercloses the gap using a 100V actuation voltage as shown from themeasurements in FIG. 32.

6. MEMS Latching Actuator

Once the MEMS mirror has been rotated to the appropriate angle foralignment it would be beneficial to lock the mirror into positionallowing the electrostatic voltage to be removed and improving theoptical integrated circuits performance against vibration and mechanicalshock, for example. Referring to FIG. 29 in first image 2900A there isdepicted an example of a latching actuator for MEMSM devices accordingto an embodiment of the invention employing mast teeth 2910 on the endof the mast 2920 (connecting beam from the rotational actuator to theMEMS mirror distal to the MEMS mirror which engage with latch teeth 2910under control of the latching actuator 2930 which provides for movementof the latch teeth 2910 when the latch locks, not depicted but depictedin FIG. 30. The latching actuator 2930 exploits electrostatic actuators2940. Second image 2900B in FIG. 29 depicts an optical micrograph of afabricated latching structure which is depicted in first image 2900A.Referring to FIG. 30 there is depicted detail of the latching lock atthe end of the latching structures 2940 in first and second images 3000Aand 3000B with an optical micrograph of the fabricated structure inthird image 3000C. FIG. 33 depicts the displacement versus electrostaticvoltage for the MEMSM latching structure such as depicted in respect ofFIG. 29 for the latching mechanism indicating 2.5 μm displacement atapproximately 80V. FIG. 34 depicts the displacement of the latch lockunder electrostatic actuation.

Accordingly, the latching actuator locks the mast position andconsequently immobilizes the mirror at a specific angle. Moreover, thisstopping action is reinforced upon activation of the gap closer throughthe torsion of the mast. Within the exemplary embodiment depicted inFIG. 29 the latch structure was developed for implementation with afabrication process having a 3 μm minimum feature size. As such thestructure comprises a curved rack with 48 tooth, where each teeth is 3μm wide and 11 μm deep and allow the mast the get inserted in betweenteeth. The mast is able to be locked into place and it can do so for 19different positions within an 8 degree coverage, which give an angularresolution of approximately 0.45 degrees. It would be evident thatalternate designs would provide for different angular resolution. Theelectrostatic actuator of the latch is using two linear racks of 18teeth of width 3 μm and length 15 μm allowing a displacement of 8 μm ofthe latch, anchored by 2 springs arms of 3.5 μm thickness and 375 μmlong including 3 spires. This structure requires an 80V actuationvoltage to lock the mast into position with a displacement ofapproximately 2.5 μm as depicted in FIG. 32.

Whilst the latching described in respect of FIG. 29 allows the rotationangle of the MEMS mirror to be locked it does not prevent resetting ofthe MEMS mirror position under failure of power to the module or failureof the electrostatic control circuit. Accordingly, as described inrespect of FIG. 30 the latch lock prevents the latch from going back toits initial position after it reaches the locked position. This allowsfor the MEMS mirror to remain in its set position once the actuationvoltage is removed from the actuators, simplifying its use under aset-and-forget usage paradigm or in order to make the system more energyefficient and able to survive power outages. Accordingly, no DC bias isrequired once the latch lock has been engaged to keep the mirror inposition.

Within the exemplary embodiment depicted in respect of FIG. 30 the latchlock was developed to be implemented with the same fabrication processhaving a 3 μm minimum feature size consistent with the other MEMSelements. The latch lock as depicted in FIG. 30 consists of a mast thatcan be moved out of the way of the latch by an electrostatic actuator atapproximately 350V, see FIG. 34. This frees the motion of the latchrelative to the latch lock. When the latch is in position, the voltageis removed and the lock physically blocks the latch into position,preventing it from springing back into its unlatched state. Thisstructure therefore allows a lock that does not require any voltage tomaintain its lock. Rather actuating the latch is only required to movethe latch lock for latch motion. The angled structure of the latch lockdoes not allow the release of the latch without latch lock actuation.

7. MEMS Pull-in Reduction

In many MEMS devices a phenomenon referred to as “pull-in” whichdescribes a failure of the device through collapse of a microbeam forexample in resonators or the failure of the spring forces within a MEMSelement to overcome the electrostatic attraction such that oppositelycharged elements snap together. Accordingly, in the prior artimplementing a MEMS spring has been viewed as one solution to the issue.However, these are typically complex structures with large footprint.

Referring to FIG. 35 there are depicted first to third anchor variations3500A to 3500C respectively according to an embodiment of the inventionwherein a MEMS anchor spring is installed at the anchor to offset theeffect of pull-in. As depicted the MEMS anchor spring may be triangular,stepped pyramidic, and dual triangle according to the required strengthand its dimensions adjusted to establish the point within the MEMSactuation that it engages as well as the dimensions of its structuremodified according to the elastic characteristics sought. Other shapesand geometries may be employed including coil, rectangular, etc.

Beneficially the MEMS anchor spring according to embodiments of theinvention provides for a simplification of the structure and reduces thefootprint compared with prior art springs on the MEMS structure.Additionally, the MEMS anchor spring reduces elastic stress and plasticdeformation of the spring as the MEMS anchor spring is only required tohandle a small displacement rather than the full displacement. It alsoreduces the risk of short-circuits when placed close to otherstructures.

8. Temperature Compensation and Control

As discussed supra in respect of FIG. 26 lateral motion under anelectrostatic comb drive can be employed to vary the optical gap betweenthe SC-MEMSM mirror and the optical waveguide(s). Whilst the gap may bereduced or closed to reduce the inherent loss present within the opticalsystem the gap may also be modulated yielding a low frequency dithersignal which may be used to adjust aspects of the operation of theoptical circuit and/or optical system incorporating the SC-MEMSM mirroraccording to embodiments of the invention. For example, the modulatedgap signal may be used to provide directly temperature compensation forexample through a signal from a temperature sensor or as an opticallybased temperature sensor to provide feedback to other control circuitsand/or elements for similarly managing the performance of the opticalcircuit and/or optical system.

This dynamic gap actuation could also be applied to others components ofthe WADER. For example, when the Si 720 and SiO2 730 are etched from thebackside using ME/DRIE processes as depicted in the tenth and eleventhschematics 1000J and 1000K in FIG. 13 and eleventh and twelfthschematics 1700K and 1700L in FIG. 20 then the Bragg reflectors may besimilarly “released” by removing the Si 720 and SiO2 730. Accordingly,the Bragg reflectors could be actuated mechanically if they weresuspended in order to allow for their tuning and thus achievetemperature compensation. Alternatively, they may be electricallyactivated through heaters wherein the substrate Si 720 removal and SiO2730 etching provide for a significant reduction in the thermal mass tobe controlled.

It would be evident that this mechanical compensation could be includedwithin a feedback loop that would essentially be using an accuratetemperature sensor to establish the correct mirror gap size and theBragg reflector deflection. This integrated control allows for a morecompact control and regulation subsystem.

Within embodiments of the invention described above in respect of FIGS.1 through 34 then it would be evident to one skilled in the art that theembodiments have been described with respect to a pair of specificapplications. However, in other embodiments of the invention:

-   -   the Bragg gratings may be employed to filter forward propagating        signals that proceed to other portions of the optical circuit        and/optical system;    -   the Bragg gratings may be employed to reflect a predetermined        portion and propagate the remainder;    -   the SC-MEMSM mirror and/or the optical circuit may couple to        free space optics rather than waveguide optical circuit        elements;    -   the SC-MEMSM mirror may scan an optical signal;    -   the Bragg gratings may be formed using other techniques than        cladding modulated first order gratings including, but not        limited to, waveguide width variations, different optical        materials, doping, ion implantation, and photoinduced refractive        index variations;    -   the Bragg gratings may be uniform, sampled, apodized, chirped,        and tilted.    -   Echelle gratings can be incorporated within the structure to        achieve diffractive mirrors;    -   other photonic integrated circuit optical filters may be        implemented for wavelength filtering such as Fabry-Perot filters        and ring resonators for example.

Within embodiments of the invention described above in respect of FIGS.1 through 34 then it would be evident to one skilled in the art that theembodiments have been described with respect to particularconfigurations. However, in other embodiments of the invention andwavelength tunable transmitters, receivers, and transceivers the Bragggratings may be:

-   -   sequential in wavelength across the device;    -   pseudo-randomly sequenced; and    -   according to a predetermined wavelength plan.

Within embodiments of the invention described above in respect of FIGS.1 through 34 then it would be evident to one skilled in the art that theembodiments have been described with respect to particularconfigurations. However, in other embodiments of the inventionconfigurations may:

-   -   exploit multiple SC-MEMSM elements for increased angular range;    -   exploit paired SC-MEMSM elements to select/deselect a specific        wavelength in different portions of an optical device;    -   exploit additional optical elements within the planar waveguide;    -   collimating/focusing transmissive grating;    -   collimating/focusing reflective grating;    -   polarizers;    -   multiple optical amplifiers coupling to multiple channel        waveguides;    -   machined waveguide lens;    -   index induced waveguide lens; and    -   waveguide Fresnel lens.

Specific details are given in the above description to provide athorough understanding of the embodiments. However, it is understoodthat the embodiments may be practiced without these specific details.For example, circuits may be shown in block diagrams in order not toobscure the embodiments in unnecessary detail. In other instances,well-known circuits, processes, algorithms, structures, and techniquesmay be shown without unnecessary detail in order to avoid obscuring theembodiments.

The foregoing disclosure of the exemplary embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many variations andmodifications of the embodiments described herein will be apparent toone of ordinary skill in the art in light of the above disclosure. Thescope of the invention is to be defined only by the claims appendedhereto, and by their equivalents.

Further, in describing representative embodiments of the presentinvention, the specification may have presented the method and/orprocess of the present invention as a particular sequence of steps.However, to the extent that the method or process does not rely on theparticular order of steps set forth herein, the method or process shouldnot be limited to the particular sequence of steps described. As one ofordinary skill in the art would appreciate, other sequences of steps maybe possible. Therefore, the particular order of the steps set forth inthe specification should not be construed as limitations on the claims.In addition, the claims directed to the method and/or process of thepresent invention should not be limited to the performance of theirsteps in the order written, and one skilled in the art can readilyappreciate that the sequences may be varied and still remain within thespirit and scope of the present invention.

What is claimed is:
 1. A device comprising: a rotatablemicroelectromechanical system (R-MEMS) formed upon a substratecomprising a platform coupled to a MEMS actuator for rotating theplatform relative to the substrate, the platform having at least a frontsurface and a back surface; a first planar waveguide disposed on theplatform supporting optical signal propagation within a predeterminedwavelength range, the planar waveguide having a first surface disposedproximate the front surface of the platform and a second surfacedisposed towards the back surface of the platform; and an opticalcircuit formed upon the substrate supporting optical signal propagationwithin the predetermined wavelength range, the optical circuit having afacet disposed adjacent to the front surface, wherein the front surface,first surface, and facet have a predetermined geometrical shape in aplane parallel to the substrate.
 2. The device according to claim 1,wherein in a first configuration: the second surface of the first planarwaveguide has the predetermined geometrical shape in the plane parallelto the substrate and reflects optical signals propagating within thefirst planar waveguide within the predetermined wavelength range; thepredetermined geometrical shape is an arc of circle; the facet and frontsurface have a common centre point for their respective arcs of circlesbut different radii to define a gap between the facet and front surface;the second surface has a predetermined radius; and the optical circuitcomprises a second planar waveguide which has the facet of the opticalcircuit formed within it and a plurality of channel optical waveguidescoupled to the second planar waveguide wherein each of channel waveguideof the plurality of channel optical waveguides is coupled to the secondplanar waveguide at a predetermined position such that it issubstantially at the predetermined radius from the second surface for apredetermined rotation of the R-MEMS; in a second configuration: thesecond surface of the first planar waveguide is a reflective dispersiveelement for optical signals propagating within the first planarwaveguide within the predetermined wavelength range; the predeterminedgeometrical shape is an arc of circle; the facet and front surface havea common centre point for their respective arcs of circles but differentradii to define a gap between the facet and front surface; and theoptical circuit comprises a second planar waveguide which has the facetof the optical circuit formed within it and a plurality of channeloptical waveguides coupled to the second planar waveguide wherein eachof channel waveguide of the plurality of channel optical waveguides iscoupled to the second planar waveguide at a predetermined position suchthat it is proximate a focus of the reflective dispersive element for apredetermined rotation of the R-MEMS; and in a third configuration: thesecond surface of the first planar waveguide has a second predeterminedgeometrical shape in the plane parallel to the substrate and reflectsoptical signals propagating within the first planar waveguide within thepredetermined wavelength range; the predetermined geometrical shape isan arc of circle; the second predetermined geometrical shape isparabolic; the facet and front surface have a common centre point fortheir respective arcs of circles but different radii to define a gapbetween the facet and front surface; and the optical circuit comprises asecond planar waveguide which has the facet of the optical circuitformed within it and a plurality of channel optical waveguides coupledto the second planar waveguide wherein each of channel waveguide of theplurality of channel optical waveguides is coupled to the second planarwaveguide at a predetermined position such that it is proximate a focusof the parabolic mirror formed by the reflective second surface for apredetermined rotation of the R-MEMS.
 3. The device according to claim1, further comprising a linear translational microelectromechanicalactuator coupled to the R-MEMS, wherein the second surface of the firstplanar waveguide has the predetermined geometrical shape in the planeparallel to the substrate and reflects optical signals propagatingwithin the first planar waveguide within the predetermined wavelengthrange; the predetermined geometrical shape is an arc of circle; thefacet and front surface have a first predetermined radius; and in afirst position the linear translational microelectromechanical actuatorestablishes a gap between the facet and front surface allowing forrotation of the R-MEMS relative to the optical circuit; and in a secondposition the linear translational microelectromechanical actuator bringsthe front surface into contact with the facet.
 4. The device accordingto claim 1, further comprising an anchor attached to the substrate;wherein the anchor is coupled to the R-MEMS between the platform and theMEMS actuator.
 5. The device according to claim 1, further comprising alinear translational microelectromechanical actuator coupled to theR-MEMS; a plurality of stoppers disposed at predetermined positionsrelative to the facet of the optical circuit; wherein the lineartranslational microelectromechanical actuator adjusts the size of a gapbetween the front surface of the platform and the facet of the opticalcircuit; and a minimum gap the between the front surface of the platformand the facet of the optical circuit is defined by predeterminedpositions of the plurality of stoppers as at the minimum gap the frontsurface of the platform abuts the plurality of stoppers.
 6. The deviceaccording to claim 1, further comprising a linear translationalmicroelectromechanical actuator coupled to the R-MEMS; at least onespring of a plurality of springs; wherein the linear translationalmicroelectromechanical actuator adjusts the size of a gap between thefront surface of the platform and the facet of the optical circuit; theMEMS actuator comprises a first non-rotating portion and a secondrotating portion; the platform and the second rotating portion of theMEMS actuator are joined solely by the at least one spring of theplurality of springs.
 7. The device according to claim 6, furthercomprising a plurality of stoppers disposed at predetermined positionsrelative to the facet of the optical circuit; wherein a minimum gapbetween the front surface of the platform and the facet of the opticalcircuit is defined by predetermined positions of the plurality ofstoppers as at the minimum gap the front surface of the platform abutsthe plurality of stoppers.
 8. The device according to claim 6, furthercomprising a plurality of stoppers disposed at predetermined positionsrelative to the facet of the optical circuit; wherein the plurality ofstoppers prevent adhesion of the platform to the portion of the devicecomprising the optical circuit.
 9. The device according to claim 1,further comprising a linear translational microelectromechanicalactuator; a mast connected to a rotating portion of the MEMS actuatordistal to the platform; a plurality of first teeth disposed on the endof the mast distal to its connection to the MEMS actuator; a pluralityof second teeth disposed on the linear translationalmicroelectromechanical actuator; wherein in a first position the lineartranslational microelectromechanical actuator disengages the secondteeth from the first teeth allowing the MEMS actuator to rotate theplatform; and in a second position the linear translationalmicroelectromechanical actuator engages a subset of the second teethwith the first teeth locking the rotation angle of the MEMS actuator andplatform relative to the optical circuit.
 10. The device according toclaim 9, further comprising a second linear translationalmicroelectromechanical actuator; wherein with the linear translationalmicroelectromechanical actuator in the second position and absent anyactuation the second linear translational microelectromechanicalactuator engages the linear translational microelectromechanicalactuator thereby locking it in the second position; and with the secondlinear translational microelectromechanical actuator actuated the lineartranslational microelectromechanical actuator can be transitioned fromthe second position to the first position.
 11. The device according toclaim 1, further comprising an anchor attached to the substrate; and ananchor spring; wherein the anchor is coupled to the R-MEMS between theplatform and the MEMS actuator; the anchor spring is attached to theanchor; and the anchor spring engages against a predetermined portion ofthe R-MEMS once the MEMS actuator has rotated past a predetermined anglein order to counter a pull-in effect arising from electrostaticactuation of the R-MEMS.
 12. The device according to claim 11, whereinthe anchor sprint at least one of: increases the elastic nature of themicroelectromechanical element once the predetermined angle has beenreached; and the anchor spring is one of triangular, stepped pyramidicdual triangular, rectangular and a coil.
 13. The device according toclaim 1, wherein the platform is formed from a first ceramic material;the first planar waveguide and optical circuit each comprise a coreformed from a second ceramic material.
 14. The device according to claim13, wherein either: the first ceramic material is silicon; the secondceramic material is either silicon carbide or silicon nitride; or thesecond predetermined ceramic material is one of silicon carbide (SiC),silicon nitride (Si₃N₄), aluminum nitride (AlN), alumina (Al₂O₃),zirconia (ZrO₂), and diamond (C).
 15. The device according to claim 1,further comprising an actuator coupled to the R-MEMS for adjusting a gapbetween the facet and the first surface; wherein the gap is modulated bya dither signal applied to the actuator; and a signal generated independence upon the modulated gap is employed by a control circuit tomanage one or more aspects of performance of an optical systemcomprising at least the optical circuit and the planar waveguide. 16.The device according to claim 1, wherein at least one of: the opticalcircuit couples to free space optical elements; and the device is one ofa pair of paired devices within an optical device.
 17. The deviceaccording to claim 1, wherein the MEMS actuator is one of a plurality ofMEMS actuators coupled to each other to increase the angular range ofrotation for at least one of a predetermined actuation signal applied tothe plurality of MEMS actuators and a maximum angular motion of theplurality of MEMS actuators.
 18. A device comprising: a rotatablemicroelectromechanical system (R-MEMS) formed upon a substratecomprising a platform coupled to a MEMS actuator for rotating theplatform relative to the substrate, the platform having at least a frontsurface and a back surface; a first optical circuit disposed on theplatform supporting optical signal propagation within a predeterminedwavelength range, the first optical circuit having a first surfacedisposed proximate the front surface of the platform and a secondsurface disposed towards the back surface of the platform; and a secondoptical circuit formed upon the substrate supporting optical signalpropagation within the predetermined wavelength range, the opticalcircuit having a facet disposed adjacent to the front surface, whereinthe front surface, first surface, and facet have a predeterminedgeometrical shape in a plane parallel to the substrate.
 19. The deviceaccording to claim 18, wherein the first optical circuit comprises aplanar waveguide; and the first optical circuit also comprises a secondsurface disposed towards the back surface of the platform.
 20. Thedevice according to claim 18, wherein a first portion of the secondoptical circuit is disposed relative to a first portion of the firstoptical circuit when the R-MEMS is rotated to a first position; and thefirst portion of the second optical circuit is disposed relative to asecond portion of the first optical circuit when the R-MEMS is rotatedto a second position.
 21. A device comprising: a rotatablemicroelectromechanical system (R-MEMS) formed upon a substratecomprising a platform coupled to a MEMS actuator for rotating theplatform relative to the substrate, the platform having at least a frontsurface and a back surface; a first optical circuit comprising at leastone optical waveguide disposed on the platform supporting optical signalpropagation within a predetermined wavelength range, the first opticalcircuit having a first surface disposed proximate the front surface ofthe platform and a second surface disposed towards the back surface ofthe platform; and a second optical circuit formed upon the substratesupporting optical signal propagation within the predeterminedwavelength range, the optical circuit having a facet disposed adjacentto the front surface, wherein the front surface, first surface, andfacet have a predetermined geometrical shape in a plane parallel to thesubstrate.
 22. The device according to claim 21, wherein a first portionof the second optical circuit is disposed relative to a first portion ofthe first optical circuit when the R-MEMS is rotated to a firstposition; and the first portion of the second optical circuit isdisposed relative to a second portion of the first optical circuit whenthe R-MEMS is rotated to a second position.